Infrared target temperature correction system and method

ABSTRACT

Infrared IR thermometer calibration systems and methods are disclosed in which the temperature of an IR thermometer calibration system is controlled such that radiation emitted by a target at a given input temperature is equal to the radiation emitted by a graybody heated to the input temperature and having an emissivity equal to an emissivity setting of an IR thermometer to be calibrated using the IR thermometer calibration system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/940,277, filed Nov. 14, 2007. This application is incorporated byreference herein in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates to systems and methods for calibrating infraredthermometer calibration systems used in the calibration of infraredthermometers.

BACKGROUND OF THE INVENTION

Infrared (IR) thermometers measure the IR radiation from bodies andoutput a temperature corresponding to the intensity of radiationmeasured within the frequency range of the IR thermometer. IRthermometers are calibrated by sensing the radiation from a thermaltarget heated to a precisely known temperature. The radiance measured bythe IR thermometer may then be mapped to the set temperature of thetarget.

The accuracy of thermal targets is limited by changes in the emissivityof the target with temperature. The emissivity of the target also varieswith the wavelength of radiation incident on and reflected from thetarget. However, the IR thermometer being calibrated may have anemissivity setting that is less than one, such that the IR thermometermay be used to measure the temperatures of bodies having lowemissivities. However, the emissivity setting is typically constant andis therefore not equal to the emissivity of the target across a range oftemperatures. Accordingly, the IR thermometer may not be accuratelycalibrated for all temperatures within a needed range.

In view of the foregoing, it would be an advancement in the art toprovide a convenient system and method for accurately calibrating an IRthermometer having a constant emissivity setting using a thermal targethaving temperature-dependent emissivity.

SUMMARY OF THE INVENTION

In one aspect of the invention, the temperature of an IR thermometercalibration system is controlled such that radiation emitted by a targetat a given input temperature is equal to the radiation emitted by agraybody heated to the input temperature and having an emissivity equalto an emissivity setting of IR thermometers to be calibrated using theIR thermometer calibration system.

In another aspect of the invention, a plurality of radiance measurementsare taken of the IR thermometer calibration system at a plurality oftemperature set points using a reference IR thermometer. The referenceIR thermometer may be calibrated using a quasi blackbody and may have anemissivity setting equal to that of IR thermometers to be calibratedusing the IR thermometer calibration system. The radiance measurementsare used to calculate correction factors mapping the temperature setpoints to apparent temperatures, the apparent temperatures correspondingto the temperature at which a graybody having an emissivity equal to theemissivity setting would emit the same amount of radiation as detectedby the radiance measurement. When an IR thermometer is calibrated usingthe IR thermometer calibration system, the user inputs a temperaturewhich is then mapped to a temperature set point using the correctionfactors. The target is then heated to the temperature set point and theradiance measured by the IR thermometer. The IR thermometer is thencalibrated to map the measured radiation to the temperature input by theuser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an IR thermometer calibration system inaccordance with an embodiment of the present invention.

FIG. 2 is a block diagram of an IR thermometer being calibrated using aquasi blackbody in accordance with an embodiment of the presentinvention.

FIG. 3 is a process flow diagram of a method for calibrating an IRthermometer calibration system in accordance with an embodiment of thepresent invention.

FIG. 4 is a block diagram of a setup for calibrating an IR thermometerusing an IR thermometer calibration system in accordance with anembodiment of the present invention.

FIG. 5 is a process flow diagram of a method for calibrating an IRthermometer in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, an IR thermometer calibration system 10 forinfrared (IR) thermometers may include a target 12 in thermal contactwith a temperature sensor 14. The target 12 may have a radiating face16. The face 16 may bear a high-emissivity coating such as ahigh-emissivity paint, which may have a temperature dependentemissivity, though the emissivity of the coating may vary less withtemperature than most substances. The sensor 14 may be positionedopposite the radiating face 16. Accordingly, the reading of the sensor14 may not indicate the exact temperature of the face 16 due to heatloss to radiation and conduction.

A heating element 18 is also positioned in thermal contact with thetarget 12. The heating element 18 may be positioned opposite theradiating face 16. The heating element 18 and sensor 14 are coupled to acontroller 20 that reads an output from the sensor 14 and controls powerto the heating element according to the reading and a set temperaturespecified by a user or program executed by the controller 20. Aninterface 22 may be coupled to the controller 20 to enable a user toinput a set temperature to the controller 20. A memory 24 coupled to thecontroller 20 may store test results and other operational andexecutable data, such as a calibration application 26.

An IR thermometer 28 may be positioned a distance from the target 12 toreceive IR radiation 30 therefrom. The IR thermometer 28 may be coupledto the controller 20 to provide a radiance measurement to the controller20. Referring to FIG. 2, while still referring to FIG. 1, in oneembodiment, the IR thermometer calibration system 10 is calibrated usingan IR thermometer 28 that is itself calibrated by a quasi blackbody 32,such as the radiation emitted from a heated chamber 34. The IRthermometer 28 preferably has an accuracy greater than that of IRthermometers to be calibrated using the IR thermometer calibrationsystem 10. The IR thermometer 28 may measure the radiation from theblackbody 32 at various temperature set points and map its output toactual radiance values from the blackbody 32 in the spectral band of theIR thermometer 28, such as 8-14 microns. When measuring radiance valuesfrom the blackbody 32, the IR thermometer preferably has its emissivitysetting set to about one.

Referring again to FIG. 1, in one embodiment, a user, or the calibrationapplication 26, inputs a series of set temperatures to the controller 20and the IR thermometer 28 measures the radiance of the target 12 at eachtemperature set point. Referring to FIG. 3, while still referring toFIG. 1, a method 36, for calibrating an IR thermometer calibrationsystem 10 may include setting a temperature T_(SET) and driving thetemperature of the target 12 to proximate T_(SET) at block 38. At block40, a contact temperature T_(CONT) is measured by reading the output ofthe temperature sensor 14. At block 42, loss correction is performed todetermine a surface temperature T_(SURF) at the face 16 of the target12. In some embodiments, an ambient temperature T_(AMB) is measured atblock 44 and used at block 42 to determine T_(SURF). DeterminingT_(SURF) may include consulting a look-up table mapping T_(SURF) to aT_(CONT) and, in some embodiments, a T_(AMB) as determined by a priorcalibration. Alternatively, T_(CONT) and optionally T_(AMB) may be inputto an equation or computational algorithm to determine T_(SURF).

At block 46 the radiance M of the target 12 for the surface temperatureT_(SURF) is measured using the IR thermometer 28. The IR thermometer 28may advantageously be calibrated as described above in relation to FIG.2. At block 48, the apparent temperature T_(APP) of the target 12 iscalculated. T_(APP) may be calculated according to Equation 1, using theexpected bandwidth λ1-λ2, emissivity setting ε_(IR), and/or backgroundtemperature setting T_(BG) of an IR thermometer to be calibrated usingthe IR thermometer calibration system 10. In some embodiments, the IRthermometer 28 used to measure the radiance M may have the samebandwidth λ1-λ2, T_(BG), and ε_(IR) settings as the units to becalibrated. In such embodiments, the IR thermometer 28 may output atemperature reading that is used as T_(APP).

At block 50, a correction factor is calculated relating T_(SURF) toT_(APP). The method 36 may be repeated for multiple T_(SET) temperaturesand their corresponding T_(SURF) temperatures to calculate a T_(APP) anda corresponding correction factor for a number of temperature setpoints.

The apparent temperature may be calculated by using Equation 1, where Mis a particular radiance measurement, ε_(IR) is the expected emissivitysetting of an IR thermometer being calibrated, T_(BG) is the expectedbackground temperature setting in degrees Kelvin of the IR thermometerbeing calibrated, and T_(APP) is the apparent temperature for theparticular radiance measurement M. Equation 1 may be solved to determineT_(APP) by any suitable numerical method known in the art.

$\begin{matrix}{M = {{\int_{\lambda \; 1}^{\lambda_{2}}{\frac{ɛ_{IR}c_{1}}{\lambda^{5}\left( {^{\frac{c_{2}}{\lambda \; T_{APP}}} - 1} \right)}\ {\lambda}}} + {\int_{\lambda \; 1}^{\lambda_{2}}{\frac{\left( {1 - ɛ_{IR}} \right)c_{1}}{\lambda^{5}\left( {^{\frac{c_{2}}{\lambda \; T_{BG}}} - 1} \right)}\ {\lambda}}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In an alternative embodiment of the method 36, the IR thermometer 28 hasan emissivity setting of one and is used to take radiance measurementsof the IR thermometer calibration system 10 at a number of settemperatures. The readings of the IR thermometer 28 are used tocalculate the emissivity of the target 12 at each temperature set point.Calculating the emissivity of the target 12 may include using Equation 1to solve for ε_(IR), where T_(SURF) for a given set point is substitutedfor T_(APP) and M is the radiance measured using the IR thermometer 28at the particular set point, and ε_(IR) is the emissivity of the target12 at the set point.

When calibrating a unit under test according to the alternative method,these experimentally determined emissivity values may be used to solveEquation 2. The temperature input by an operator is T_(APP) and theactual temperature needed to provide the radiation of a perfect graybodyat T_(APP) is calculated using the experimentally determined emissivityvalues of the target 12 by using Equation 2. In Equation 2, ε_(TGT) isthe emissivity of the target at or near a temperature set point,determined as described above. In some embodiments, the value of ε_(TGT)is determined by interpolating between emissivity values determinedexperimentally as described above or by using a curve fit equation basedon the experimentally determined emissivity values. ε_(IR) is theemissivity setting of the unit under test (typically approximately equalto 0.95), T_(BG) is the background temperature, and T_(APP) is theapparent temperature corresponding to a particular set temperatureT_(TGT). Equation 2 may be solved to determine T_(APP) by any suitablenumerical method known in the art.

$\begin{matrix}{{{\int_{\lambda \; 1}^{\lambda_{2}}{\frac{ɛ_{TGT}c_{1}}{\lambda^{5}\left( {^{\frac{c_{2}}{\lambda \; T_{TGT}}} - 1} \right)}\ {\lambda}}} + {\int_{\lambda \; 1}^{\lambda_{2}}{\frac{\left( {1 - ɛ_{TGT}} \right)c_{1}}{\lambda^{5}\left( {^{\frac{c_{2}}{\lambda \; T_{BG}}} - 1} \right)}\ {\lambda}}}} = {{\int_{\lambda \; 1}^{\lambda_{2}}{\frac{ɛ_{IR}c_{1}}{\lambda^{5}\left( {^{\frac{c_{2}}{\lambda \; T_{APP}}} - 1} \right)}\ {\lambda}}} + {\int_{\lambda \; 1}^{\lambda_{2}}{\frac{\left( {1 - ɛ_{IR}} \right)c_{1}}{\lambda^{5}\left( {^{\frac{c_{2}}{\lambda \; T_{BG}}} - 1} \right)}\; {\lambda}}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Referring to FIG. 4, a unit under test (UUT) IR thermometer 52 having anemissivity setting 54 and a background temperature setting 56 may becalibrated using the IR thermometer calibration system 10 as calibratedaccording to the method of FIG. 3.

Referring to FIG. 5, a method 58 for calibrating the UUT IR thermometer52 may include calibrating a reference IR thermometer, such as by usingthe quasi blackbody 32 to calibrate the IR thermometer 28, at step 60.At step 62, correction factors relating temperature set points toapparent temperatures are determined, such as by the method 36. At step64 a calibration temperature is input to the IR thermometer calibrationsystem 10 as the temperature set point. At step 66, one or more of thecorrection factors are applied to the calibration temperature by meansof direct mapping, interpolation between calculating factors for testedset temperatures bounding the calibration temperature, or by applying acurve fit equation determined according to the correction factorsdetermined according to the method 36. The correction factors relate thecalibration temperature to a corrected temperature for the face 16. Thecorrected temperature is the temperature at which the radiation emittedby the face 16 will be about equal to the radiation emitted by agraybody having an emissivity equal to the emissivity setting of the UUTIR thermometer 52 and heated to the calibration temperature. The IRthermometer calibration system 10 then heats the target face 16 to thecorrected temperature at step 68. Step 68 may include calculating aT_(CONT) value for the temperature sensor reading that corresponds tothe face 16 being at the calibration temperature and heating the face 16such that T_(CONT) is equal to a value corresponding to a facetemperature T_(SURF) approximately equal to the calibration temperature.

At step 70, the radiance of the target is measured with the UUT IRthermometer 52. At step 72 the UUT IR thermometer is calibrated tooutput the calibration temperature when subject to the radiance measuredat step 70. The UUT IR thermometer 52 may then measure radiance fromother bodies and output a temperature corresponding to the radiation ofthe bodies according to the temperature calibration at step 72.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. A calibration system comprising: an infrared thermometer having anemissivity setting; a target emitting infrared radiation toward theinfrared thermometer; a heating element in thermal contact with thetarget; a temperature sensor in thermal contact with the target; and acontroller coupled to the temperature sensor and heating element, thecontroller configured to receive an input temperature, the controllerprogrammed to map the input temperature to a set temperature and tocause the heating element to heat the target to proximate the settemperature, the set temperature corresponding to an apparenttemperature of the target equal to the input temperature, the apparenttemperature being substantially equal to a graybody temperature of agraybody having an emissivity substantially equal to the emissivitysetting of the infrared thermometer and having a radiance substantiallyequal to the target when heated to the set temperature.
 2. Thecalibration system of claim 1, wherein the target has a high-emissivitycoating.
 3. The calibration system of claim 1, wherein the emissivitysetting is less than one.
 4. The calibration system of claim 3, whereinthe emissivity is between about 0.9 and one.
 5. The calibration systemof claim 4, wherein the emissivity is between about 0.9 and 0.95.
 6. Amethod for calibrating an infrared target comprising: taking a pluralityof first radiance measurements of an infrared target, each of theplurality of radiance measurements being taken with the infrared targetheated to one of a plurality of target set temperatures; for each firstradiance measurement calculating an emissivity value of the infraredtarget; inputting a calibration temperature into a controller coupled tothe infrared target; calculating a set temperature within the controlleraccording to the calibration temperature and at least one of theemissivity values, the target emitting infrared radiation at the settemperature that is approximately equal that of a graybody having anemissivity setting of a unit under test (UUT) infrared thermometer andheated to the calibration temperature; heating the target to the settemperature; taking a second radiance measurement of the target with theUUT infrared thermometer; and calibrating the UUT infrared thermometerto map the radiance measurement to the calibration temperature.
 7. Themethod of claim 6, wherein taking the plurality of first radiancemeasurements of the infrared target comprises taking a plurality offirst radiance measurements of the infrared target with a referenceinfrared thermometer calibrated according to a quasi blackbody.
 8. Themethod of claim 7, wherein the UUT and reference infrared thermometershave substantially identical frequency response bands.
 9. The method ofclaim 6, wherein the target has a high emissivity coating.
 10. Themethod of claim 6, wherein the emissivity setting is less than one. 11.The method of claim 10, wherein the emissivity is less than or aboutequal to 0.95.
 12. The method of claim 6, further comprising measuring aradiance of a subject using the UUT infrared thermometer and outputtinga temperature corresponding to the step of calibrating the UUT infraredthermometer.
 13. The method of claim 6, further comprising deriving aplurality of contact temperatures for each of the plurality of settemperatures, the plurality of contact temperatures each correspondingto an output of a sensor in thermal contact with a target when thesurface of the target is at the set temperature; and wherein the step ofheating the target to one of the plurality of set temperatures comprisesheating the target such that the temperature sensor produces an outputcorresponding to the contact temperature corresponding to one of theplurality of set temperatures.